Abstract Less than 10% of metastatic tumors are curable by current therapies, a fact that warrants the need for more effective strategies to treat these tumors. The overall goal of this project is to develop a nanoscale drug carrier system to improve therapy of cancer metastases. The applicant of this K99/R00 proposal recently designed a novel polypeptide drug carrier, chimeric polypeptides (CP), which self-assembles into nanoparticles upon drug conjugation, and which displays a long circulation half-life and good accumulation in tumors, as compared to free drug. The candidate hypothesizes that the invasive behaviors of metastasis-initiating cells (MICs) can be utilized for drug delivery using CP-based nanoparticles loaded with an anti-MIC drug, salinomycin (Sali) base on the following observations: (a) nanoscale drug carriers accumulate in the perivasuclar region of tumors; (b) MICs migrate through the perivascular space and invade a blood vessel as their first step in metastasis, and (c) MICs rely on potent peptidase activities in the local extracellular matrix (ECM) to degrade the ECM facilitate their migration. We will leverage these three facts to design a nanoscale delivery system that specifically targets MICs via a CP nanoparticle that is loaded with Sali. The overall hypothesis of this proposal will be tested by the following three aims: (1) Sali will be conjugated with a range of CPs with varied composition, physico-chemical properties and molecular weights to systematically vary the in vivo stability of the CP-Sali conjugate; (2) the attachment triggered self-assembly of the CP-Sali conjugates into sub-100 nm diameter particles and their in vivo stability will be quantified; (3) a peptide substrate of an invasion-associated proteinase, matrix metalloproteinase 2 (MMP2), will be incorporated into the primary amino acid sequence of the CP, and the resulting MMP2-dependent cleavage, cellular uptake, cytotoxicity, and the metastasis- inhibitory activity of CP(MMP2)-sali conjugates will be studied. The proposed drug carrier system will be the first to exploit the mobility of MICs for drug delivery, and the study may lead to a novel therapy for cancer metastases. The overall career goal of the candidate is to become an independent investigator contributing at the interface of nanotechnology and cancer therapy. This goal is backed by candidate's excellent prior training and research productivity. Through this career development award, the candidate will: (1) acquire additional training under the mentorship of Dr. Ashutosh Chilkoti and Dr. Mark W. Dewhirst, who are well-known investigators in nanotechnology and cancer therapy, respectively; (2) closely interact with his career advisory committee and accomplish career transition under the guidance of the committee; (3) produce research results, which serve not only as a foundation for him to apply for future federal funding on cancer nanotechnology. This research is relevant to public health because it will lead to an innovative therapeutic strategy for the treatment of cancer metastasis.